“Rosan” style or AS1986 fittings and similar fittings are commonly used in aerospace applications, to connect fluid lines to a block or body, for example. A first end of the fitting is attached to a fluid line and the second end is placed into a bore in the body and secured thereto in a well-known manner. As illustrated in FIG. 6, these fittings may take the form of a fitting 200 having a main body portion 201 having an axial centerline 202, a seating surface 204 angled at an angle α relative to axial centerline 202 and extending between a sidewall 203 and a shoulder 205, a neck 206 projecting from shoulder 205, and a threaded end portion 208. Such fittings also generally include an internal bore 209 for carrying a fluid and an O-ring (not shown) surrounding at least a portion of neck 206 to better seal the fitting.
The fittings may be placed into bores, such as bore 210, which includes a bore seat 212 and a threaded portion 214 complementary to the threads of the threaded end portion 208 of fitting 200. In use, the threaded end portion 208 of the fitting is inserted into bore 210 until it reaches threaded portion 214 of bore 210. Fitting 200 is then rotated to threadedly engage threaded end portion 208 and bore threads 214 and draw fitting 200 into bore 210. This action moves seating surface 204 of fitting 200 toward and ultimately into contact with bore seat 212. After seating surface 204 contacts bore seat 212, a user continues to apply torque to fitting 200 until a desired preload is attained. Fluid fittings having this form are generally known; fittings without internal bores could alternately be provided to perform connecting or joining functions without carrying a fluid. As used herein, the term “fastener” includes versions of the fitting described above both with and without internal bores.
The bore into which a fastener is inserted is frequently formed in a material (such as aluminum) different than the material used to form the fastener (such as titanium). These materials have different coefficients of thermal expansion, and thus the preload on current fasteners tends to decrease with increasing temperature due to axial slip at the angled seating surface. A reduced preload at elevated operating temperatures and/or repeated changes to the preload may lead to premature fatigue failure of the fitting or the threads. It might be possible to address this problem by carefully matching the thermal properties of the fastener and the block. However, for various reasons, including cost and performance, this is often not possible or practical. It would therefore be desirable to control the preload on a fastener so that the preload remains substantially constant with changing temperature or varies with changing temperature in a desired manner.